The present invention relates generally to inductive loops intended for vehicle detection and more particularly to surface mounted inductive loops intended for vehicle detection and more particularly to strip material for forming such loops.
Inductive loops are well known in the art for use in vehicle detection. See for example, U.S. Pat. No. 3,989,932, Koerner, entitled Inductive Loop Vehicle Detector; U.S. Pat. No. 3,984,764, Koerner, entitled Inductive Loop Structure; and U.S. Pat. No. 3,943,339, Koerner, Inductive Loop Vehicle Detector. Generally these loops consist of one of more loops (turns) of a conductive wire embedded under the surface of a roadway over which the vehicle to be detected passes. For example, the inductive loop may be embedded in the street to detect vehicles in the roadway approaching an intersection. Typically, a six foot (1.8 meters) across circular or nearly square loop is utilized along with a lead wire leading from the loop to the side of the roadway. The loop may then be connected via the lead wire to well known devices which typically oscillate the loop at a predetermined frequency and monitor changes in the frequency in order to detect the presence of a vehicle over the inductive loop. Examples of these well known devices are described in the above-referenced patents.
These typical prior art loops which are embedded in the roadway require the cutting of the roadway in order to install the loops. Typically saw cuts are made in the roadway to allow for the installation of the loops. Saw cuts impair the integrity of the roadway and facilitate entry of moisture which may dilatoriously affect the roadway, e.g. during freeze-thaw-freeze cycles, and jeopardize the strength of the roadway by creating a "weak" spot which may facilitate uneven settling or shifting of the roadway under heavy vehicular loads. In some installations it may be impossible to saw cut the surface of the roadway on which the inductive loop is to be installed. An example where this may be the case is a parking ramp constructed from precast, prestressed concrete slabs where the saw cutting of the slabs may seriously weaken the structure.
A few inductive loops exist which are intended for mounting on the surface of a roadway. When surface mounted, the loop is subject to the stress of the vehicular traffic over it, whether it be high speed, heavy truck traffic in a roadway application or turning wheels in a parking application. Further, the surface mounted loop is subject to exposure to the elements in the form of rain, snow and corrosive materials. Still further, the surface mounted loop is subject to the uneven shifting in the roadway and the resultant uneven surface of the roadway to which the loop must conform.
Golden River Corporation of 7315 Redfield Court, Falls Church, Va. has manufactured a surface mounted inductive loop. The loop is a temporary surface loop for vehicle counting and classifying. The loop uses an adhesive, a protective ribbon, a loop wire, plugs and siliconized scissors for installation. The loop uses a standard continuous metallic conductor for forming the loop conductor and hence is relatively inextensible and subject to breakage on stretch as may be caused by the uneven surface or shifting of a roadway.
A device known as an INDUKTIONSPLATTE produced by GEO Merkel GmbH and Co. KG of West Germany uses a preformed detector loop built into a plastic plate and is indicated as suitable for mounting on the surface of a roadway. The plastic plate is relatively inflexible and utilizes a conductor of a uniform copper with a wire size of approximately number AWG 25.5.
Universal Autopayment, Ltd. of Morley Road, Tonbridge, Kent manufactures an inductive loop surface tape which has been designed for use as a substitute for buried loops. The tape comprises 21 specially designed copper strands which provide a multiplicity of conductive paths allowing the loop to continue to function if some of the conductive paths are broken.
These prior art surface mounted inductive loops attempt to solve the breakage problem either by preventing deformation (INDUKTIONSPLATTE) or by providing a multiplicity of paths (Universal Autopayment) such that if some breakage occurs the loop would still function.